1. Field of the Invention
Embodiments of the invention described herein pertain to the field of liquefaction of natural gas. More particularly, but not by way of limitation, one or more embodiments of the invention describe systems and methods for the dockside liquefaction of natural gas on a floating unit.
2. Description of the Related Art
Natural gas is typically transported by pipeline from the location where it is produced to the location where it is consumed. However, large quantities of natural gas may sometimes be produced in an area or country where production far exceeds demand, and it may not be feasible to transport the gas by pipeline to the location of commercial demand, for example because the location of production and the location of demand are separated by an ocean or rain forest. Without an effective way to transport the natural gas to a location where there is a commercial demand, opportunities to monetize the gas may be lost.
Liquefaction of natural gas facilitates storage and transportation of the natural gas. Liquefied natural gas (“LNG”) takes up only about 1/600 of the volume that the same amount of natural gas does in its gaseous state. LNG is produced by cooling natural gas below its boiling point (−259° F. at atmospheric pressure). LNG may be stored in cryogenic containers slightly above atmospheric pressure. By raising the temperature of the LNG, it may be converted back to its gaseous form.
The demand for natural gas has stimulated the transportation of LNG by special vessels. Natural gas produced in locations where it is abundant, may be liquefied and shipped overseas in this manner to locations where it is most needed. Typically, the natural gas is gathered through one or more pipelines to a land-based liquefaction facility. Land-based liquefaction facilities and the associated gathering pipelines are costly, may occupy large areas of land and take several years to permit and construct. Thus, land-based facilities are not optimally suited to adapt to variation in the location of natural gas supplies or to liquefy small or stranded gas reserves. In addition, once natural gas is liquefied at a land based facility, the LNG must be stored in large land-based cryogenic storage tanks, transported through a special cryogenic pipeline to a terminal facility, and then loaded onto a vessel equipped with cryogenic compartments (such a vessel may be referred to as an LNG carrier or “LNGC”), which in combination may increase the overall expense of transporting the gas to its ultimate destination.
In some instances, natural gas deposits may be found in underwater gas fields located in the open ocean, such as locations more than 100 miles to the nearest land. In such situations it has been proposed that natural gas be liquefied on large offshore floating platforms that are turret moored or spread moored to the bottom of the sea, and located above the well head in the open ocean. These floating liquefaction vessels are large in size, typically about 450 or 500 meters from stern to bow, since they must be fully-integrated, self-contained gas processing and LNG production facilities: all gas processing, liquefaction equipment, cooling systems, condensate storage and waste storage must be included onboard. Such arrangements are costly due to the size of the facilities, the difficulties of working in the open ocean, and extensive subsea infrastructure requirements in order to extract the gas and transfer it to the offshore platform or vessel in order to be liquefied and transported. Therefore, this offshore, fully-integrated approach is often not practical or economical for use with small or stranded natural gas reserves located offshore, reserves located near shore, or on land reserves.
Conventional techniques for liquefying natural gas are not well suited for small or stranded natural gas reserves located offshore, natural gas reserves located near shore, or on land natural gas reserves, as they are not cost effective and are slow to market. Therefore, there is a need for systems and methods for floating dockside liquefaction of natural gas.